Droplets size distributions

Emulsion A has a droplet size distribution that obeys the ordinary Gaussian error curve. The most probable droplet size is 5 iim. Make a plot of p/p(max), where p(max) is the maximum probability, versus size if the width at p/p(max) = j corresponds to... 

As an example figure B 1.14.13 shows the droplet size distribution of oil drops in the cream layer of a decane-in-water emulsion as determined by PFG [45]. Each curve represents the distribution at a different height in the cream with large drops at the top of the cream. The inset shows the PFG echo decay trains as a fiinction of... 

Figure Bl.14.13. Derivation of the droplet size distribution in a cream layer of a decane/water emulsion from PGSE data. The inset shows the signal attenuation as a fiinction of the gradient strength for diflfiision weighting recorded at each position (top trace = bottom of cream). A Stokes-based velocity model (solid lines) was fitted to the experimental data (solid circles). The curious horizontal trace in the centre of the plot is due to partial volume filling at the water/cream interface. The droplet size distribution of the emulsion was calculated as a fiinction of height from these NMR data. The most intense narrowest distribution occurs at the base of the cream and the curves proceed logically up tlirough the cream in steps of 0.041 cm. It is concluded from these data that the biggest droplets are found at the top and the smallest at the bottom of tlie cream.

MoDonald P J, Ciampi E, Keddie J L, Fleidenreioh M and Kimmioh R, Magnetio resonanoe determination of the spatial dependenoe of the droplet size distribution in the oream layer of oil-in-water emulsions evidenoe for the effeots of depletion floooulation Rhys. Rev. E, submitted... 

Droplet Size Distribution. Most sprays comprise a wide range of droplet sizes. Some knowledge of the size distribution is usuaUy required, particularly when evaluating the overaU atomizer performance. The size distribution may be expressed in various ways. Several empirical functions, including the Rosin-Rammler (25) andNukiyama-Tanasawa (26) equations, have been commonly used. 

In agricultural spraying, one of the biggest concerns is the drifting of small droplets. Drifting sprays not only lead to waste and environment problems, but also could endanger other nearby crops. Droplets smaller than 150 p.m can be easily blown away from the intended target area by a cross wind. A typical herbicide atomizer produces a spray with 15—20% of the Hquid volume contained in droplets less than 150 p.m. Atomizer improvements must be made so that the spray contains a narrow droplet size distribution with Hquid volume less than 5% contributed by the smaller droplets. 

The foHowing general expression (eq. 22) is commonly used to describe the droplet size distribution in a spray ... 

Droplet Size Distribution Instead of the single droplet size imphed by the discussion above, a spectrum of droplet sizes is produced. The most common ways to characterize this spectrum are ... 

FIG. 14-88 Droplet -size distribution for three different types of nozzles. To convert pounds per square inch gauge to Idlopascals, multiply by 6.89 to convert gallons per minute to cubic meters per hour, multiply by 0.227. (Spraying Systems Inc. )... 

FIG. 14-90 Entrainment droplet-size distribution. To convert meters per second to feet per second, multiply by 3.28, to convert meters to feet multiply by 3.28. 

Air atomizing nozzles are commonly used to control the droplet-size distribution independently of the liquid feed rate and to minimize the chances of defluidization due to uncontrolled growth or large droplets. 

Product diameter is small and bulk density is low in most cases, except prilling. Feed hquids must be pumpable and capable of atomization or dispersion. Attrition is usually high, requiring fines recycle or recoveiy. Given the importance of the droplet-size distribution, nozzle design and an understanding of the fluid mechanics of drop formation are critical. In addition, heat and mass-transfer rates during... 

Droplet size distribution Design of treatment/recovery Numerous including ... 

Water-sensitive papers are readily available in most countries and provide a convenient system for visually assessing spray drift performance. These papers are coated with bromoethyl blue, which turns from yellow to blue when contacted with water. " However, since any water can cause this change in color, care needs to be taken to prevent accidental exposure to sources of water other than the pesticide. Such cards do not work well under humid conditions, and are not appropriate for sampling droplets with diameter below 15 qm. Quantitative estimates of droplet size distributions must take account of the exponential increase in droplet volume as the droplet diameter increases. As droplets strike the paper, the liquid spreads over the surface and leaves a stain with a size that is dependent on the volume of the droplet. The apparent droplet size will be greater for large droplets than for small droplets, and the size determination must be corrected to avoid bias. 

A Malvern Mastersizer (Malvern Instruments Ltd, Malvern, UK) with optical parameters defined by the manufacturer s presentation code 0505 was used to determine the droplet size distribution. The measurement was made in triplicate at room temperature. Water was used to disperse the emulsion droplets. 

In terms of measuring emulsion microstructure, ultrasonics is complementary to NMRI in that it is sensitive to droplet flocculation [54], which is the aggregation of droplets into clusters, or floes, without the occurrence of droplet fusion, or coalescence, as described earlier. Flocculation is an emulsion destabilization mechanism because it disrupts the uniform dispersion of discrete droplets. Furthermore, flocculation promotes creaming in the emulsion, as large clusters of droplets separate rapidly from the continuous phase, and also promotes coalescence, because droplets inside the clusters are in close contact for long periods of time. Ideally, a full characterization of an emulsion would include NMRI measurements of droplet size distributions, which only depend on the interior dimensions of the droplets and therefore are independent of flocculation, and also ultrasonic spectroscopy, which can characterize flocculation properties. 

Figure 4.5.8 shows the spatially-resolved droplet size distribution of a fully creamed isooctane-in-water emulsion obtained using the PGSTE pulse sequence shown in Figure 4.5.5(b). It can be seen that this method is able to provide droplet size distributions with spatial resolution. 

The flow artifacts detected in the droplet size measurements are similar to those reported by Goux et al. [79] and Mohoric and Stepisnik [80]. In their work natural convection effects led to an increase in the decay of signal attenuation curves, causing over-prediction in the self-diffusion coefficient of pure liquids. In order to avoid flow effects in droplet size distributions, flow compensating pulse sequences such as the double PGSTE should be used. It has been demonstrated recently that this sequence facilitates droplet size measurements in pipe flows [81]. 

Studies of flow-induced coalescence are possible with the methods described here. Effects of flow conditions and emulsion properties, such as shear rate, initial droplet size, viscosity and type of surfactant can be investigated in detail. Recently developed, fast (3-10 s) [82, 83] PFG NMR methods of measuring droplet size distributions have provided nearly real-time droplet distribution curves during evolving flows such as emulsification [83], Studies of other destabilization mechanisms in emulsions such as creaming and flocculation can also be performed. 

Spatially-resolved measurement of the droplet size distribution can be accomplished by the implementation of velocity compensated pulse sequences, such as the double PGSTE [81] in a spatially resolved imaging sequence. Accurate measurements of spatially resolved droplet size distributions during flow and mixing of emulsions would provide truly unique information regarding flow effects on the spatial distribution of droplets. 

G. J. W. Goudappel, J. P. M. van Duyn-hoven, M. M. W. Mooren 2001, (Measurement of oil droplet size distributions in food oil/water emulsions by time domain pulsed field gradient NMR), /. Colloid Interface Sci. 239, 535. 

M. Heidenreich, R. Kimmich 1999, (Magnetic-resonance determination of the spatial dependence of the droplet size distribution in the cream layer of oil-in-water emulsions Evidence for the effects of depletion flocculation) Phys. Rev. E 59, 874. 

K. J. Packer, C. Rees 1972, (Pulsed NMR studies of restricted diffusion. 1. Droplet size distributions in emulsions),/. Colloid Interface Sci. 40, 206. 

Techniques of emulsification of pharmaceutical products have been reviewed by Block [27]. The location of the emulsifier, the method of incorporation of the phases, the rates of addition, the temperature of each phase, and the rate of cooling after mixing of the phases considerably affect the droplet size distribution, viscosity, and stability of the final emulsion. Roughly four emulsification methods can be distinguished ... 

Rheological Property Determination. The rheology of an emulsion is often an important factor in determining its stability. Any variation in droplet size distribution, degree of flocculation, or phase separation frequently results in viscosity changes. Since most emulsions are non-Newtonian, the cone-plate type device should be used to determine their viscosity rather than the capillary viscometer. 

Droplet radius, in polymer blends, 20 333 Droplet size correlations, 23 190-191 Droplet size distribution, in polymer blends, 20 332-333 Droplet sizes, in sprays, 23 185 Drop-on-demand (DOD) inkjet printing, 9 222... 

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